The study of self-assembled ZnO nanorods grown on Si(111)
by plasma-assisted molecular beam epitaxy
A.J. Tzou
a, K.F. Chien
a, H.Y. Lai
a, J.T. Ku
a, L. Lee
a, W.C. Fan
a, W.C. Chou
a,b,na
Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan
bNSC Taiwan Consortium of Emergent Crystalline Materials, Taiwan
a r t i c l e
i n f o
Available online 8 January 2013 Keywords:
A1. Nanostructures A3. Molecular beam epitaxy B1. Oxides
B1. Zinc compounds
B2. Semiconducting II–VI materials
a b s t r a c t
The growth and optical properties of self-assembled ZnO nanorods grown on Si(111) substrate by plasma-assisted molecular beam epitaxy (PA-MBE) were studied. By controlling the Zn/O flux ratio, the growth of ZnO nanorods on Si(111) substrate without catalyst has been achieved. Scanning electron microscopy (SEM) shows the ZnO nanorods with various density and diameter could be controlled. Photoluminescence (PL) measurements exhibit nice optical properties. The sharp near band edge PL emission with full width at half maximum (FWHM) of about 17 meV indicates that the ZnO nanorods could be used as the high efficient photonic devices.
Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction
ZnO has attracted lots of attention due to its superior elec-trical, optical and piezoelectric properties for the application in light emitting diodes, solar cells and surface acoustic wave devices etc. [1–4]. Compared with two-dimensional epitaxy layers, ZnO nanorod structure has significantly reduced threading dislocation density[5]. Stark effect can be suppressed by growing quantum well structures on the sidewall of c-axis oriented nanorod [6]. Therefore, high quality ZnO nanorod of designed structure is important for the application in photonic devices.
The growth of ZnO nanorod has been achieved by vapor– liquid–solid (VLS) method[7], nanoimprint[8]and catalyst-free metal organic chemical vapor deposition (MOCVD)[9]. However, growth of catalyst-free self-assembled ZnO nanorod by MBE was rarely studied[10]. In this article, the growth of high quality ZnO nanorods on Si(111) substrates without catalyst by PA-MBE was accomplished. The crystal structure properties were studied in more details by transmission electron microscopy (TEM) cross-section images and X-ray reciprocal space mapping (RSM).
2. Experiments
ZnO nanorods were grown on Si(111) substrates by using PA-MBE. Silicon substrates were cleaned by 10% HF to remove
the native oxide. Before the growth, the substrate was desorbed at 8301
C until Si(111)(7 7) pattern was observed on reflection high energy electron diffraction (RHEED) screen. The substrate temperature was decreased to 650 1C for the deposition of Zn layer for 10 minutes to prevent the formation of amorphous silicon oxide on the desorbed Si(111) surface. The oxygen plasma of 1.5 105Torr was then turned on with the oxygen flow rate of 0.6 sccm and RF plasma power of 250 W. The Zn flux was adjusted from 1.00 108to 8.61 108Torr to vary the Zn/O flux ratio.
The surface morphology and density were analyzed by JEOL JSM-7001F scanning electron microscope (SEM). A high resolution JEOL ARM-200F TEM was employed to study the crystalline properties. The energy dispersive X-ray spectroscopy (EDS) was carried out during the TEM observation to probe the Zn and Si composition distribution in nanorods. The crystal structure was also analyzed by RSM with synchrotron at beam line BL-17B1 in the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The PL spectra were excited by the 325 nm line of a He–Cd laser and analyzed by using a iHR-550 monochromator and LN2cooling CCD.
3. Results and discussion
InFig. 1, the SEM images of ZnO nanorods grown with different Zn/O ratios were shown. The nanorod diameter increases with increasing Zn flux. For Zn flux above 5.86 108Torr, ZnO nanorods
merged, as shown inFig. 1(c) and (d). As the Zn flux reaches to 8.61 108Torr, the rods poorly aligned with irregular shape.
Nanorod density and diameter as functions of Zn flux are schemati-cally shown inFig. 2.
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Journal of Crystal Growth
0022-0248/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.12.137
n
Corresponding author at: National Chiao Tung University, Department of Electrophysics, Hsin-Chu 30010, Taiwan.
E-mail addresses: [email protected] (A.J. Tzou), [email protected] (W.C. Chou).
The cross-section TEM images of ZnO nanorods grown at Zn flux of 3.38 108Torr are shown inFig. 3(a)–(c). InFig. 3(a), amorphous
SiO2was observed between the Si substrate and ZnO nanorods. The
existence of amorphous SiO2layer implies that the pre-growth of Zn
layer has no effect to prevent the formation of silicon dioxide at the initial growth stage. Because of ZnO compounds have weaker bonding than SiO2 and it is more difficult to crystallize at high
growth temperature during the initial stage[11]. It is confirmed by comparing the formation enthalpy of SiO2(910.771.0 kJ/mol) and
ZnO ( 350.4670.27 kJ/mol)[12]. InFig. 3(b), there exists a mixed compound layer of about 4 nm between the SiO2 amorphous layer
and ZnO nanorods. As shown inFig. 3(c), the atomic arrangement of mixed compound layer gradually changes from amorphous to crystal-line structure. The constitute elements of this mixed compound layer can be determined by EDS point scan, as shown inFig. 4. The Cu
signal is from the copper holder. The Zn composition gradually increases from the bottom (amorphous SiO2 side) of mixed
com-pound layer. Whereas, the Si composition decreases when EDS scan moves away from the amorphous SiO2. This result implies the
inter-diffusion between Zn and Si[13].
Depth profile RSM of ZnO nanorods was investigated to explore the strain variation in the vertical orientation of crystal structure. In
Fig. 5(a), the l axis of reciprocal lattice unit (r.l.u.) from 0.02 to 0.16, which is equivalent to scan X-ray from surface to interface, was plotted versus h axis (r.l.u.). The intensity shifted slightly in h axis as the scan depth increasing. The h reciprocal lattice unit could be transferred to 2
y
degree and schematically shown inFig. 5(b). In the interfacial region, 2y
diffraction peak situates at 31.661, which implies tensile strain. When the X-ray beam probed the surface, l¼ 0.02, 2y
degree shifted to 31.761. It implies that the strain is released in the surface region. Thus, this result indicated the ZnO a-axis lattice constant was released from 3.2604 ˚A to strain free 3.2508 ˚A, 0.3% strain was released[14].
Fig. 6(a) shows the PL spectrum for ZnO nanorods at 10 K. The strong and sharp near-band edge (NBE) emission peak was observed. The PL emission peak consists of neutral donor bound exciton (D1X) at 3.357 eV and free exciton (FXA) at 3.374 eV[15–16].
In the inset ofFig. 5(a), no deep-level emission peak was observed. The narrow FWHM for the NBE peak, near 17 meV, implies superior quality of the nanorods.Fig. 5(b) shows the temperature-dependent PL measurement. With increasing temperature, the intensity of the D1X peak decreases and the FXA peak dominates along with a simultaneous energy red shift.
4. Conclusions
In summary, growth of self-assembled ZnO nanorods on Si(111) by PA-MBE was achieved. By TEM cross-section image, we can confirm that the well aligned nanorods crystalline structure lies on
Fig. 1. SEM images of ZnO nanorods grown on Si(111) at various Zn flux: (a) 1.99 108Torr, (b) 3.38 108Torr, (c) 5.86 108Torr, and (d) 8.61 108Torr.
Fig. 2. The density and diameter of ZnO nanorod structure grown at various Zn flux.
Fig. 3. TEM cross-sectional image of ZnO nanorods grown on Si(111) substrate: (a) low magnification image, (b) and (c) high resolution images of interface and ZnO nanorods, respectively.
Fig. 4. (a) HRTEM cross-sectional image of ZnO nanorods grown on Si(111) substrate. EDS point scans at the positions of (b) 1, (c) 2, (d) 3, and (e) 4 ofFig. 4(a).
Fig. 5. Depth profile reciprocal space mapping of ZnO nanorods grown on Si(111) substrate. (a) Reciprocal space mapping of l and h axis. (b) 2yX-ray scan for various l values plot.
A.J. Tzou et al. / Journal of Crystal Growth 378 (2013) 466–469 468
amorphous SiO2 and a very thin Zn/Si mixed compound layer.
The RSM depth profile indicates the strain relaxation of near 0.3% from interface to surface of nanorods. The PL spectra show well optical emission properties. It implies that the nanorods could be used for the further growth of ZnO/ZnMgO nanorod multi-quantum well (MQW) with high emission efficiency.
Acknowledgments
This work was supported by the National Science Council under the Grant number of NSC100-2119-M-009-003.
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Fig. 6. PL spectra of ZnO nanorods: (a) low temperature PL spectrum, full range PL spectrum is in the inset and (b) temperature-independent PL spectra. A.J. Tzou et al. / Journal of Crystal Growth 378 (2013) 466–469 469
